1. Field of the Invention
The present invention relates to an imaging system and, more specifically, to an electronic imaging system equipped with a sequential exposure rolling shutter.
2. Description of Related Art
In recent years, it is a noticeable trend to employ complementary metal-oxide semiconductor (CMOS) image sensor devices for electronically capturing images of subjects in electronic imaging systems in light of the advantages of low cost, small size and low electric power consumption. Such a CMOS image sensor device generally comprises an image sensor array of photodetectors, one for each pixel, arranged in two-dimensional and a rolling electronic shutter or circuitry, in place of a mechanical shutter, for readout of the pixel signals. The photodetector for each pixel in the image sensor array converts photons impinging on the pixel to an electric charge via the photoelectric effect. The charge is integrated over a period of time that is long enough to collect a detectable amount of charge but short enough to avoid saturating storage elements. This period of time is analogous to a film exposure time or shutter speed and is called an integration time. In the image sensor array of the CMOS image sensor device, the integration time is the time between a reset of a given row and a subsequent read of the row. Because only one row can be selected at a time, the reset/read routine is sequentially repeated row by row (sequential exposure control). This sequence is referred to as a “rolling electronic shutter,” or simply as a “rolling shutter.” As shown in FIG. 10, the rolling shutter accomplishes exposure by sequentially reading succeeding rows, one row at a time, from top to bottom of a frame with certain delays to provide electronic image signals. On the other hand, electronic flash devices are most popular as means for lighting subjects to be photographed Such an electronic flash device is such that, when a high voltage stored in the capacitor is applied to the gas-discharge tube, the gas inside becomes ionized and then a rush of current flows through the gas the tube and it simultaneously emits a bright light by sudden discharge. In general, as shown in FIG. 10, the rolling shutter allows a flash to be generated for an overlapping period of charge integration (exposure) of all the rows of the image sensor array of the CMOS image sensor device for uniform exposure of a flame.
Meanwhile, cellular phones with an image import feature, that have recently been in widespread use, are generally provided with a single white light emitting diode (white LED) built therein as a flash or lighting unit for illuminating subjects to be imported. In general, installation of the single white LED is advantageous to the purpose of space-saving. The cellular phone image import system is designed such that, in the event of importing a picture under, for example, weaker room illumination than daylight, while a first button is kept operated to excite the white LED for illuminate of an aimed subject, a second button is subsequently operated for import of an image of the subject after fixing composition of the image. When the import of the image is completed, the white LED is extinguished.
In the imaging system equipped with a CMOS image sensor device including a rolling shutter, exposure can be made with a plurality of flashes as described in Japanese Unexamined Patent Publication No. 2002-359774. When making exposure using flashes, the exposure may be varied in condition so as to decrease incoming radiation intensity using a desired ND filter as described in Japanese Unexamined Patent Publication No. 2002-350964. The lighting unit that comprises a single white LED element is known in various forms such as a lens-fitted film unit equipped with a lighting unit disclosed, for example, in Japanese Unexamined Patent Publication No. 11-133490. Another example of the lighting unit that is disclosed in Japanese Unexamined Patent Publication No. 11-258363 is such that an LED is excited upon switch-on operation for opening and closing a mechanical shutter and remains excited while the mechanical shutter is kept opened.
One of the problems that the imaging system equipped with a rolling shutter encounters is that, although flashes can be enabled to affect charge integration (exposure) of all the rows of the image sensor array that overlap one another for a certain period of time when the shutter speed is low as shown in FIG. 11A, a flash is only allowed to affect charge integration (exposure) of some adjacent pixel rows of the image sensor array when the shutter speed is high as shown in FIG. 11B, because it is impossible for the image sensor array that all the rows have periods of charge integration overlapping one another. In consequence, the image sensor array of the CMOS image sensor device causes a difference in exposure between the rows which a flash affects and the rows free of influence of a flash This exposure difference makes speeding up of the rolling shutter of the image CMOS image sensor device.
When designing light and thin, more compact electronic imaging systems with built-in lighting unit, it is more advantageous to employ white LEDs as compared with an electronic flash tube. However, the lighting unit comprising a single white LED element has the problem that light the single white LED element emits is too low in radiation intensity to make correct exposure in a dark scene. It is one approach to a solution of the problem to use a plurality of white LEDs in the lighting unit that are simultaneously excited. However, this simultaneous excitation of white LEDs causes another problem that electric power consumption increases according to the number of excited white LEDs. In particular, in the case where the single white LED is installed in cellular phone having an optical communication feature in addition to the image import feature, even if the white LED can have radiation intensity, the cellular phone will be disabled to make calls or data communication due to a potential drop of batteries resulting from image import and is off from practical use.
Nothing in the above referenced publications is disclosed regarding lighting control of an LED array installed in an electronic imaging system equipped with the sequential exposure rolling shutter. It is impossible to apply the teachings of Japanese Unexamined Patent Publication No. 11-258363 that the lighting control of the LED is made in connection with switch-on operation for opening a mechanical shutter to the imaging system disclosed in Japanese Unexamined Patent Publication No. 2002-359774 that is equipped with a CMOS image sensor device including a rolling shutter. This is because there is an essential distinction of physical structure and exposure mechanism between the mechanical shutter for blanket exposure and the rolling shutter for sequential exposure. Further, there is no suggestion in the referenced publications how to coordinate sequential exposure by the rolling shutter and lighting control of the LED together.